The developers, which have conventionally widely been used for developing a photopolymerizable PS plate for use in making lithographic printing plate, can roughly be divided into the following three groups a) to c): a) a non-aqueous developer mainly comprising an organic solvent; b) an aqueous developer mainly comprising an inorganic alkali; and c) an aqueous developer mainly comprising an organic base.
Among them, there have recently been used the aqueous developers b) and c) because of the requirements for the environmental protection.
The characteristic properties of these two kinds of developers will be detailed below. The inorganic alkali-containing developer b) is characterized in that it in general comprises a silicate at a pH value in the proximity to 12 for the purpose of the hydrophilization of the surface of the substrate after the completion of the development.
This silicate is an essential component used in the hydrophilization treatment for preventing any staining of the non-image area of the lithographic printing plate during printing operations.
For instance, there have been known a developer having a pH value of not less than 12 such as those disclosed in Japanese Un-Examined Patent Publication (hereunder referred to as “J.P. KOKAI”) Hei 8-248643 and a developer having a pH value of not more than 12 such as those disclosed in J.P. KOKAI Hei 11-65129. The former developer having a pH value of not less than 12 may easily dissolve aluminum commonly used as a material for a substrate. In particular, in case of a point having a small image area (a dot), there is observed such a phenomenon that the aluminum substrate just under the dot image is dissolved due to the side etching phenomenon and such dot is eliminated from the substrate during printing operations (skipping over of dots). In other words, the developer suffers from a problem such that it may significantly deteriorate the printing durability of the resulting lithographic printing plate.
In addition, the latter developer having a pH value of not more than 12 is excellent in the foregoing printing durability and resistance to staining during printing, but the developer suffers from a new problem such that if the developing treatment using the developer is continued over a long period of time, the pH value of the developer is liable to be reduced due to, for instance, the action of carbon dioxide present in the air and the silicate is separated out from the developer and this makes the developing treatment unstable.
Moreover, there have also been proposed developers free of any silicate and examples thereof are disclosed in, for instance, J.P. KOKAI Nos. Sho 61-109052, 2000-81711 and Hei 11-65126 and German Patent No. 1,984,605. However, these developers free of any silicate are inferior in the resistance to staining during printing as compared with the silicate-containing developers and it is difficult for the developers to simultaneously satisfy the requirements for the resistance to staining during printing and the printing durability.
On the other hand, there have been known, as the organic base-containing developer c), those comprising an organic amine such as ethanolamine and an alcoholic organic solvent such as benzyl alcohol as an auxiliary developing agent. In this case, the pH value thereof is certainly low on the order of about 10 and therefore, these developers are not adversely affected by carbon dioxide and thus ensure stable processing of exposed PS plates. However, these developers are disadvantageous in the hydrophilization of the substrate, while their ability of penetrating into the image areas is too strong, in particular, they adversely affect dot images and the dot images are removed from the substrate during development or they suffer from a problem of deteriorating the image-forming ability of the PS plate for making lithographic printing plates.
In other words, there has not yet been developed any satisfactory developer, while taking into consideration the problems concerning, for instance, the image-forming ability, staining of the printing plate during printing operations and printing durability and the ability of stable processing or development in case of the photopolymerizable lithographic printing plate.
Moreover, it has been recognized that the development is greatly influenced by the developer composition and more specifically by, for instance, the presence of a silicate, the height of the pH value and the difference between the inorganic and organic alkaline agents, but the conventional combinations have never solved the foregoing problems.
Furthermore, when the photopolymerizable light-sensitive lithographic printing plate is continuously developed over a long period of time, impurities are accumulated, coagulated and precipitated in a developer used to thus generate scum formed during development (developing scum) and this in turn becomes a cause of making the development unstable.
The components of such developing scum have not yet been clearly elucidated, but it has been found, as a result of analysis, that the majority of the components are components contained in the light-sensitive layer and insoluble in the developer.
Examples of such components insoluble in the developer are various kinds of coloring agents added to the light-sensitive layer for the discrimination of light-exposed areas from un-exposed areas. As such coloring agents, there have been used, in the photopolymerizable light-sensitive layer, so-called pigments, which are not molecularly dispersed in the light-sensitive layer, but are dispersed therein in the form of aggregates, while maintaining the crystalline condition, for the purpose of preventing any desensitization due to radical-trapping and/or that due to unnecessary interaction (such as energy transfer and electron transfer) with initiator systems (such as initiators per se or combinations of sensitizing dyes with initiators). However, such pigments are essentially insoluble in the developer. Accordingly, they are initially temporarily dispersed in a developer in the developing process for removing un-exposed areas, but they are accumulated, coagulated and precipitated in the developer to thus generate developing scum, if the developing treatment is continued over a long period of time.
In addition, the photopolymerizable lithographic printing plate comprises a variety of photopolymerization initiators (radical generators) for coping with the laser light sources recently developed and there have been known, in particular, titanocene type initiators, which are sensitive to laser light beams and excellent in both stability and sensitivity. However, the titanocene type initiator is an organometal, insoluble in developers like the foregoing coloring agents or pigments. Accordingly, they are initially temporarily dispersed in a developer, but they are likewise accumulated, coagulated and precipitated in the developer to thus generate developing scum, if the developing treatment is continued over a long period of time.
It is thus quite important to solve the problem of such developing scum in addition to the solution of the foregoing problem of the silicate-precipitation associated with the reduction of the pH value in order to ensure the ability of stable processing or development.
On the other hand, as a means for improving the printing durability of a printing plate, there has been proposed the use of urethane polymer, as a binder having high strength, such as those disclosed in, for instance, J.P. KOKAI Nos. Hei 3-287165 and Hei 4-204453. However, either of them could not satisfactorily show its function if it is used in combination with the foregoing developer or they could not provide any printing plate having satisfactory printing durability.
As has been described above in detail, there has been desired for the development of a presensitized plate for use in making lithographic printing plates, which can satisfy the requirements for the image-forming ability, the simultaneous achievement of high resistance to staining and printing durability of the resulting printing plate and the stable processing ability as well as a method for making a lithographic printing plate.